1. Field of the Invention
The present invention relates, generally, to the art of oxygen sensors. In particular, the present invention relates to a method for pulse width modulating an oxygen sensor to quickly heat the sensor to within operating temperature range and maintain this range for all subsequent operating conditions of the vehicle.
2. Description of the Related Art
Oxygen sensors are typically used in a vehicle's exhaust system to sense varying amounts of oxygen so that the fuel to air ratio of exhaust gasses emanating from the engine can be calculated by an engine controller or Electronic Control Unit (ECU). When the vehicle runs rich, i.e. a large amount of fuel is present in comparison to air, large amounts of hydrocarbons (HC) are present in the vehicle exhaust. In contrast, when the vehicle runs lean, i.e. a large amount of air is present in comparison to fuel, large amounts of nitrous oxide (NOX) may be present in the vehicle exhaust. NOX is the main ingredient of ground level ozone that is commonly referred to as "smog." Carbon monoxide (CO) is present in both of the above operating conditions. Oxygen sensors are typically placed in the exhaust system of the vehicle, one upstream and one downstream of the catalytic converter, so that the operating efficiency of the catalytic converter in purifying the HC, NOX, and CO gases may be monitored via the ECU.
The oxygen sensors have a specific operating temperature range, and may not detect proper amounts of oxygen prior to reaching this range given inherent sensor limitations. Therefore, it is desirable to have the sensors quickly heated, after start-up, to within the operating temperature range thereby allowing for peak operation and oxygen detection. It has been demonstrated that the oxygen sensors will begin to be heated by the heat of the engine exhaust. Since the exhaust is relatively cool upon start of the engine or at idle, the oxygen sensors may actually be cooled by the engine exhaust. This has caused many in the industry to time delay when the ECU can go closed loop, i.e. when the ECU will take readings from the oxygen sensors. This time delay can be as long as 45 seconds after startup.
The industry has tried to remedy the operating limitations of oxygen sensors by providing other means of heating the oxygen sensors. This has typically been accomplished by equipping oxygen sensors with low resistance heating elements that are in contact with the inner core element of the oxygen sensor. Since heater performance is inversely proportional to the resistance. The lower the resistance, the quicker the heater will reach its desired temperature operating range. If the heater is too low in resistance, however, the heater will ramp to high temperature that is out of its operating range. As a result, the inner core element of the sensor may crack or otherwise have a degradation in performance by not giving accurate readings to the ECU.
Still other oxygen sensor systems supply maximum electrical power to the single heater for a set time after start-up and regulate the power thereafter. This requires the use of complex and costly circuitry for regulating voltage supplied to the heater. Wave shaping circuitry is also required for shaping current waveforms supplied to the sensor heater.
It is therefore desirable in the art of oxygen sensors to use a single low resistance heating element that quickly heats a sensor to within operating temperature range and maintains this range for all subsequent operating conditions of the vehicle.